The present invention relates to an internal combustion engine exhaust gas purifying system having a continuous regenerating diesel particulate filter (hereafter referred to as DPF) for collecting particulate matter (hereafter referred to as PM) of a diesel internal combustion engine and purifying exhaust gas.
The quantity of PM permitted to be exhausted from a diesel internal combustion engine has been more strictly regulated, together with NOx, CO, and HC emissions, year by year. Therefore, a technique for collecting the PM with a filter referred to as a DPF and reducing the quantity of the PM to be exhausted to the outside has been developed.
A PM-collecting DPF includes a ceramic such types as a monolith-honeycomb wall-flow filter and a fibrous filter formed by fibrous ceramic or metal. An exhaust gas purifying system using one of these DPFs is set in the middle of the exhaust passage of an internal combustion engine the same as the case of other exhaust gas purifying systems to purify and exhaust the exhaust gas produced in the internal combustion engine.
However, clogging of the PM-collecting DPF progresses as more PM is collected, and the exhaust gas pressure rises proportionally to the quantity of collected PM. Therefore, because of the necessity of removing PM from the DPF, several methods and systems have been developed.
As such systems, the following have been proposed: a system for regenerating a filter by using two exhaust passages and alternately collecting PM and burning the collected PM with a DPF set to each exhaust passage and a continuous regenerating system for oxidizing and removing the collected PM by forming one exhaust passage and controlling the internal combustion engine with the purpose of regenerating the filter while collecting the PM with a DPF set to the exhaust passage.
These continuous regenerating type systems include a continuous regenerating DPF type referred to as a CRT (Continuously Regenerating Trap) provided with an oxidation catalyst on the upstream side of a DPF system and a DPF type referred to as a CSF (Catalyst Soot Filter) for burning PM with an exhaust gas by lowering the combustion temperature of the PM in accordance with the action of a catalyst carried by a filter.
The CRT-type continuous regenerating DPF system, for example as disclosed in the Japanese Patent Laid-Open No. 2002-106327, exploits the fact that PM is oxidized with nitrogen dioxide at a lower temperature than oxidizing the PM with oxygen in an exhaust gas, which is constituted of an oxidation catalyst and a filter. The PM is removed by oxidizing nitrogen monoxide (NO) in an exhaust gas on the upstream-side oxidation catalyst carrying platinum or the like to produce nitrogen dioxide (NO2) and oxidizing the collected PM in a downstream-side filter with the nitrogen dioxide (NO2) to produce carbon dioxide (CO2).
Moreover, the CSF-type continuous regenerating DPF system is constituted of a catalyst-provided filter with such a catalyst as cerium oxide (CeO2). Furthermore, at a low temperature (300° to 600° C.), PM is oxidized utilizing a reaction such as (4CeO2+C→2Ce2O3+CO2, 2CeO3+O2→4CeO2) using oxygen in the exhaust gas in the catalyst-provided filter. When PM is kept in a high temperature zone higher than the temperature at which the PM is burned with oxygen in an exhaust gas (such as 600° C. or higher), the PM is oxidized with oxygen in the exhaust gas.
However, in the case of a low exhaust temperature or an operation of low nitrogen monoxide (NO) exhausted, also in these continuous regenerating DPF systems, a catalyst temperature is lowered and thereby a catalyst activity is deteriorated or nitrogen monoxide (NO) runs short. Therefore, the above reaction does no occur. Therefore, because a filter cannot be regenerated by oxidizing PM, deposition of the PM on the filter is continued and the filter is clogged.
Therefore, in the case of these continuous regenerating DPF systems, regeneration control is performed by estimating the quantity of the deposited PM in accordance with the differential pressure between upstream and downstream of a DPF when regenerating the filter, changing the present operation state of an internal combustion engine to a regenerating-mode operation when the differential pressure exceeds a predetermined judgment value. In the regeneration control, the collected PM in a filter is oxidized and removed by forcibly raising the exhaust temperature or increasing the quantity of nitrogen monoxide (NO) or nitrogen dioxide (NO2).
In the case of a conventional continuous regenerating DPF system, regeneration control is performed in which the timing for starting the regenerating-mode operation is judged according to when closing of the filter as progressed and the quantity of an estimated deposited-PM or a differential pressure becomes a predetermined judgment value or more and the regenerating-mode operation being started independently of the operation state of the internal combustion engine at the time of the above judgment.
However, when the quantity of collected PM at or before start of the regenerating-mode operation is close to its upward-limit quantity, internal combustion engine operating conditions may be affected. Moreover, when a large-load operation or full-load is started while the quantity of collected PM is close to the upward-limit value and the flow rate of exhaust gas is greatly increased, the problem that the differential pressure ΔP between the upstream and downstream of a DPF is greatly raised as shown in FIG. 6 occurs.
When the differential pressure ΔP is greatly raised, the service life of the DPF is shortened because deterioration of the DPF is accelerated, and the great rise of the differential pressure ΔP is also disadvantageous from the viewpoint of fuel efficiency. Moreover, when using a large-capacity DPF system capable of allowing a sudden rise in the exhaust gas flow rate, a problem occurs in layout or cost.